1. Field of the Invention
The present invention relates to a process for preparing 2,3-dimethyl-2,3-butanediol (pinacol).
2. Discussion of Prior Art
It is known to prepare diols by hydroxylating olefins with hydrogen peroxide in the presence of metal oxides as catalysts. According to another variant of this reaction, the unsaturated compounds are reacted with hydrogen peroxide in the presence of acids, preferably carboxylic acids. In this case the percarboxylic acid formed under the reaction conditions acts as a hydroxylating agent. In the hydroxylation, esters are first of all formed, which are then hydrolyzed to the corresponding diol in a second reaction step.
Normally the corresponding percarboxylic acid is prepared from hydrogen peroxide and acid in a separate reaction before the actual hydroxylation, and is then reacted with the olefin.
Processes that avoid the separate preparation of the peracid and use instead a mixture of hydrogen peroxide and carboxylic acid from which the relevant peracid is formed, are simpler.
The afore-mentioned processes have the disadvantage that they always involve the formation of free peracids in a fairly high concentration. Since peroxides and olefins lead to formation of explosive mixtures, these processes have proven unsuitable on safety grounds for use on a commercial scale. Moreover, the high peracid concentration reduces the yield of end product since the glycol initially obtained reacts further with excess peracid to produce undesired by-products.
The problem therefore arose of providing a process which prepares 2,3-dimethyl-2,3-butanediol in a simple and safe manner, and which ensures that the diol is obtained in high yields.